1. Field of the Invention
The present invention relates to an ATM (asynchronous transfer mode) radio transmission apparatus which recovers data from ATM information included in a received radio signal or converts input data into ATM information and transmits it as a radio signal.
2. Description of the Related Art
In a broad band ISDN (B-ISDN), the ATM (which is a transfer mode for rapidly transmitting a large volume of information) has recently been adopted in place of a conventional synchronous transfer mode (STM), and an ATM radio transmission apparatus for radio-transmitting data by using the ATM has been developed.
An outline of the ATM is described in "Pictorial Text for ATM & Multi-media", chapter 2, published by OHM Co., Ltd.
A conventional ATM radio transmission apparatus comprises (1)a radio transmission unit for receiving a signal transmitted at a radio frequency, demodulating and channel-decoding the signal to obtain received date, and for channel-coding transmission data and performing modulation transmit the transmission data over a radio frequency, (2)an ATM unit for extracting desired received ATM information from the received data, and for preparing the transmission data on the basis of transmission ATM information, and (3)an ATM unit terminal such as a personal computer (PC) for displaying the data included in the received ATM information and for outputting the transmission ATM information to the ATM unit.
The ATM unit extracts cells of the received data to recover data and inserts cells into the transmission data. The cell is a unit of a block of information handled by the ATM and has a length of 53 bytes. Each cell comprises a cell header representing a cell attribute and user information.
In order to extract the cell of the received data, it is necessary to hold cell synchronization, that is, to locate the header of the cell from the received bit sequence and identify the cell having the 53-byte length.
An operation for holding the cell synchronization in the ATM unit is explained in the above-referenced article "Pictorial Text for ATM and Multi-media", FIG. 2-9.
In order to hold the cell synchronization, a "self-synchronization" function by an HEC (Header Error Control) is used. Three states transit before the cell synchronization is established. First, in "hunting state" in which a head of the cell is not known, a CRC of the header is calculated and the calculation is repeated by shifting the received bit sequence one bit by one bit until the remainder becomes the "0". When the remainder of the CRC becomes "0", the state becomes "pre-synchronization state". In the pre-synchronization state, the HEC of the header is calculated for every 53 bytes from the time point when the remainder first becomes "0" to determine the position of the cell. When the cell header is continuously identified six times as normal, the state becomes the "synchronization established state" or "synchronization state". If the correct HEC is not continuously identified six times, the process immediately returns to the "hunting state".
In the "synchronization established state", since the head position of the cell is identified, the HEC is calculated based thereon for every 53 bytes, and if it is correct, the synchronization established state is maintained. Even if an HEC error is detected in the course of this process, it is not determined as out-of-synchronization unless the error is continuously detected seven times, and the synchronization established state is maintained. When the HEC error is continuously detected seven times, it is immediately determined as out-of-synchronization and the process returns to the "hunting state" to start the identification of the head position of the cell.
The operation of not immediately causing the state to transit even if the condition change occurs is commonly referred to as "protection", and a period until shifting to the synchronization state is referred to as "backward protection" and a period until shifting out of the synchronization state is referred to as "forward protection".
Thus, in ATM unit, the processes for the following three states are conducted:
(1) In the "hunting state", the HEC calculation is effected by shifting the temporary 32-bit header one bit by one bit. PA1 (2) In the "pre-synchronization state", the HEC calculation is effected for every 53 bytes. PA1 (3) In the "synchronization state", the HEC calculation is effected for every 53 bytes.
However, in the process performed by the ATM unit, the HEC calculation needs a very high speed of operation of 156 Mbps and power consumption is large. Therefore, a problem exists in that the ATM radio transmission apparatus including the ATM unit is required to significantly reduce the power consumption in order to be conformed to a battery-driven system.